Optical disc, tracking controlling apparatus and method, and focus controlling apparatus and method

ABSTRACT

An optical disc  1  has spiral grooves GR formed on a data recording surface thereof and in which information is written to, and read from, both a land between the grooves and the grooves as tracks. Each track has a plurality of address areas AR 1  and user areas AR 2  formed thereon, and each of the address areas AR 1  has embossed pits for a groove header GRH and embossed pits for a land header LH. Each of the address areas AR 1  has provided in a leading part thereof a header mark area HM formed from a mirror finished surface. That is, this optical disc is of an on-land/in-groove recording type. An offset taking place in a tracking error signal and focus error signal is removed to accurately control the tracking and focus of an optical head.

This is a division of Ser. No. 09/277,440 filed Mar. 26, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disc having spiral groovesformed on a data recording surface thereof and in which information iswritten to, and read from, both a land between the grooves and thegrooves as tracks, a tracking controlling apparatus and method for theoptical disc, and a focus controlling apparatus for the optical disc.

2. Description of Related Art

Various optical recording media, disc- and card-shaped, adopting anoptically or magneto-optically signal recording/reproducing method, havebeen developed and are commercially available. These optical recordingmedia include a read-only memory (ROM) type recording medium such asso-called compact disc (CD), well known as CD-ROM, a write-once typerecording medium to which the user can write data once, a rewritable(spo-called overwrite) type recording medium such as magneto-opticaldisc (MO), etc.

The optical disc drive to write and read data to and from a disc-shapedrecording medium uses an optical head incorporating a laser diode toemit a light beam for writing and reading information to and from theoptical disc, a photodetector to detect a reflected light of the lightbeam focused on the optical disc, etc. and is adapted for servo controlof the focus and tracking of the optical head based on a reflected lightdetection output provided from the photodetector. In the optical discdrive, the optical head writes and reads data to and from the opticaldisc by scanning the light beam on a recording track on the optical discwhile driving to spin the optical disc at a constant angular velocity orconstant linear velocity by a spindle motor running at aservo-controlled speed. For example, the tracking is servo-controlled bythe one-beam pushpull method or three-beam method, for example. However,taking in consideration the recent highly increased storage capacity andrecording density of optical discs, highly improved control of the laserdiode output power and optical system, and related improvements in theart, the pushpull method should desirably be used for servo control ofthe optical head tracking.

In the optical disc drive adopting the pushpull method, a light beamreflected and diffracted by grooves on the optical disc is detected bytwo photodetectors disposed symmetrically with respect to the trackcenter, and a difference between outputs from the photodetectors isdetected as a pushpull signal. The pushpull signal is used as a trackingerror for servo control of the biaxial actuator, etc. for tracking ofthe optical head.

Recently, ROM (read-only memory) and RAM (random access memory) discsproposed as optical recording media show a considerably large storagecapacity. Semiconductor lasers of shorter wavelengths have been proposedfor use in the optical heads of optical disc drives, and objectivelenses of a larger numerical aperture (NA) have been proposed for use tofocus a light beam onto the information recording surface of an opticaldisc.

To implement a phase-change type optical disc having a larger storagecapacity and higher density than DVD-RAM, it is known to use a smallerfocused light spot size. The size of a focuses spot on the recordingmedium is given generally as λ/NA. The focused spot size can be reducedby using a short-wave semiconductor laser such as GaN, ZnSe or the likeor by using a two-group lens or the like, typically, a solid immersionlens (SIL) to increase the objective lens NA.

Assume here that λ=640 nm and NA=0.85, for example. The diameter of alaser spot focused on the recording medium will be about 0.75 μm. Inthis case, when signal is written using RLL (1, 7) for example, arecording density of about 0.21 μm/bit can be attained.

Well-known modulation codes in a modulation system using a wide channeldetection window suitable for a high-density recording and reproductioninclude RLL (1, 7) and RLL (2, 7).

Further, in an optical disc system adopting an optical head in which anobjective lens of a large NA is used, to accommodate an error caused bythe influence on the light beam of dust or scratch on the surface of theoptical, the error correction capability should be improved byincreasing the code and ECC block.

Assume here that the optical disc spins without run-out. As shown inFIG. 1(A) for example, a photodetector 70 is irradiated at the centerthereof with a reflected light of the laser beam. However, if theoptical disc spins with a run-out, the photodetector 70 will beirradiated at a position off the center thereof with the reflected lightof the laser beam as shown in FIGS. 1(B) to 1(D). Thus, a DC offset willtake place in a pushpull signal produced from the reflected light of thelaser beam from the groove. FIG. 2(A) shows a pushpull signal offset,indicated with a solid line, from an ideal pushpull signal indicatedwith a dashed line.

In an optical disc in which data is written on lands as well as ingrooves, the reflected light of the laser beam from the lands andgrooves results in a pushpull signal suffering from an offset. As inFIG. 2(A), a pushpull signal produced from a reflected light of thelaser beam from the land is offset from an ideal pushpull signal asshown in FIG. 2(B).

If a pushpull signal offset takes place, no accurate tracking ispossible. Especially in a large storage capacity, high recording densityoptical disc, since the space between land and groove is decreasedcorrespondingly to the increase of the storage capacity and recordingdensity, tracking of the optical head has to be done more accurately.

In some of so-called on-land/in-groove recording type optical discs inwhich both the land track and groove track have a generally same widthand the groove depth is ⅙ to ⅕ of the wavelength of a write/read laserlight, an embossed pits are used as an address.

In writing or reading information to or read from an on-land track, forexample, however, since the polarity of a tracking error signal in anaddress area is reversed, a tracking tried with the laser spot in anaddress area after seeking cannot be performed. The actuator makes a bigjump, the laser spot is far from a target address, and thus the accesstime is longer.

Even with the laser spot focused on a track, a tracking error will be adisturbance when the optical head is in an address area, and also afocus error will be a disturbance when the optical head is in theaddress area.

Furthermore, in an on-land/in-groove recording, a focus error offset ona land track is detected to be different from a one a groove track insome cases.

Also, in an optical head using a one-beam optical system, a trackingerror offset caused by a light intensity off the center of on thephotodetector when the optical disc runs out, is a problem.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome theabove-mentioned drawbacks of the prior art by providing an optical disc,a tracking controlling apparatus and method, and a focus controllingapparatus and method, in which an offset arisen in a tracking errorsignal and focus error signal is removed to accurately control thetracking and focus of the optical head.

The above object can be achieved by providing an optical disc havingspiral grooves formed on a data recording surface thereof and in whichinformation is written to, and read from, both a land between thegrooves and the grooves as tracks,

each track having formed thereon a plurality of address areas and userareas;

each of the address areas having formed thereon embossed pits for agroove

header and embossed pits for a land header; and

each of the address areas having provided in a leading part thereof aheader mark area formed from a mirror finished surface.

The above object can be achieved also by providing an optical dischaving spiral grooves formed on a data recording surface thereof and inwhich information is written to, and read from, both a land between thegrooves and the grooves as tracks,

each track having formed thereon a plurality of address areas and userareas;

each of the address areas having formed thereon embossed pits for agroove

header and embossed pits for a land header; and

each of the address areas having provided in a leading part thereof aheader mark area formed from a mirror finished surface and grooves.

According to the present invention, the optical disc may have, forinstance, groove tracks and land tracks formed to wobble at a singlefrequency.

According to the present invention, the optical disc may be formed, forinstance, to have a plurality of zones formed by dividing the datarecording surface of the optical disc concentrically and a plurality ofsectors formed by dividing each zone on the data recording surface intoradial areas.

The above object can be achieved also by providing a trackingcontrolling apparatus, for use with an optical disc having spiralgrooves formed on a data recording surface thereof and in whichinformation is written to, and read from, both a land between thegrooves and the grooves as tracks, each track having formed thereon aplurality of address areas and user areas, each of the address areashaving formed thereon embossed pits for a groove header and embossedpits for a land header, and each of the address areas having provided ina leading part thereof a header mark area formed from a mirror finishedsurface, the apparatus comprising, according to the present invention;

means for generating a tracking error signal from a detection outputprovided from an optical head which irradiates a laser beam to, anddetects a reflected light of the irradiated laser beam from, a datarecording surface of the optical disc;

means for detecting a header mark area of the address area based on thedetection output provided from the optical head;

means for sampling and holding, based on the detection output providedfrom the header mark area detecting means, a tracking error signalgenerated by the tracking error generating means from the detectionoutput provided from the optical head staying in the header mark area ofthe address area;

means for detecting a difference between the tracking error signalgenerated by the tracking error generating means and the tracking errorsignal sampled and held by the sampling and holding means; and

means for controlling the tracking of the optical head using as atracking error signal the difference detection output provided from thetracking error difference detecting means.

The above object can be achieved also by providing a trackingcontrolling apparatus, for use with an optical disc having spiralgrooves formed on a data recording surface thereof and in whichinformation is written to, and read from, both a land between thegrooves and the grooves as tracks, each track having formed thereon aplurality of address areas and user areas, each of the address areashaving formed thereon embossed pits for a groove header and embossedpits for a land header, and each of the address areas having provided ina leading part thereof a header mark area formed from a mirror finishedsurface and grooves, the apparatus comprising, according to the presentinvention;

means for generating a tracking error signal from a detection outputprovided from an optical head which irradiates a laser beam to, anddetects a reflected light of the irradiated laser beam from, a datarecording surface of the optical disc;

means for detecting a header mark area of the address area based on thedetection output provided from the optical head;

means for sampling and holding, based on the detection output providedfrom the header mark area detecting means, a tracking error signalgenerated by the tracking error generating means from the detectionoutput provided from the optical head staying in the header mark area ofthe address area;

means for detecting a difference between the tracking error signalgenerated by the tracking error generating means and the tracking errorsignal sampled and held by the sampling and holding means; and

means for controlling the tracking of the optical head using as atracking error signal the difference detection output provided from thetracking error difference detecting means.

According to the present invention, the tracking controlling apparatusmay further comprise, for example, a tracking servo pull-in controllingmeans which disables a tracking servo pull-in with the optical headstanding in the address area and enables only a tracking servo pull-inwith the optical head standing in the user area, based on the detectionoutput provided from the header mark area detecting means.

According to the present invention, the tracking controlling apparatusmay be adapted, for instance, such that based on the detection outputprovided from the header mark area detecting means, the trackingcontrolling means controls the tracking of the optical head using as atracking error signal a sampled and held output provided from thesampling and holding means which holds the difference detection outputprovided from the difference detecting means for a period during whichthe optical head stays in the address area.

According to the present invention, the tracking controlling apparatusmay further comprise, for example, a lowpass filter means which issupplied with the difference detection output provided from thedifference detecting means;

the tracking controlling means controlling the tracking of the opticalhead using as a tracking error signal an output provided from thelowpass filter means.

The above object can be achieved also by providing a trackingcontrolling method, for use with an optical disc having spiral groovesformed on a data recording surface thereof and in which information iswritten to, and read from, both a land between the grooves and thegrooves as tracks, each track having formed thereon a plurality ofaddress areas and user areas, each of the address areas having formedthereon embossed pits for a groove header and embossed pits for a landheader, and each of the address areas having provided in a leading partthereof a header mark area formed from a mirror finished surface, themethod comprising, according to the present invention, the steps of:

generating a tracking error signal from a reflected light detectionoutput provided from an optical head which irradiates a laser beam to,and detects a reflected light of the irradiated laser beam from, a datarecording surface of the optical disc;

detecting a header mark area of the address area based on the reflectedlight detection output provided from the optical head;

sampling and holding, based on the detection output provided from theheader mark detecting means, a tracking error signal generated from thereflected light detection output from the optical head staying in theheader mark area of the address area;

detecting a difference between the tracking error signal and the sampledand held tracking error signal; and

controlling the tracking of the optical head using the differencedetection output as a tracking error signal.

The above object can be achieved also by providing a trackingcontrolling method, for use with an optical disc having spiral groovesformed on a data recording surface thereof and in which information iswritten to, and read from, both a land between the grooves and thegrooves as tracks, each track having formed thereon a plurality ofaddress areas and user areas, each of the address areas having formedthereon embossed pits for a groove header and embossed pits for a landheader, and each of the address areas having provided in a leading partthereof a header mark area formed from a mirror finished surface andgrooves, the method comprising, according to the present invention, thesteps of:

generating a tracking error signal from a reflected light detectionoutput provided from an optical head which irradiates a laser beam to,and detects a reflected light of the irradiated laser beam from, a datarecording surface of the optical disc;

detecting a header mark area of the address area based on the reflectedlight detection output provided from the optical head;

sampling and holding, based on the header mark area detection output, atracking error signal generated from the reflected light detectionoutput provided from the optical head staying in the header mark area ofthe address area;

detecting a difference between the tracking error signal and the sampledand held tracking error signal; and

controlling the tracking of the optical head using the differencedetection output as a tracking error signal.

According to the present invention, the tracking controlling method maybe, adapted, for instance, such that based on the header mark areadetection output, a tracking servo pull-in with the optical headstanding in an address area is disabled and only a tracking servopull-in with the optical head standing in the user area is enabled.

According to the present invention, the tracking controlling method maybe, for example, such that based on the header mark area detectionoutput, a tracking of the optical head is controlled with a trackingerror signal of which the difference detection output has been held fora period during which the optical head stays in the address area.

The above object can be achieved also by providing a focus controllingapparatus, for use with an optical disc having spiral grooves formed ona data recording surface thereof and in which information is written to,and read from, both a land between the grooves and the grooves astracks, each track having formed thereon a plurality of address areasand user areas, each of the address areas having formed thereon embossedpits for a groove header and embossed pits for a land header, and eachof the address areas having provided in a leading part thereof a headermark area formed from a mirror finished surface, the apparatuscomprising, according to the present invention:

means for generating a focus error signal from a detection outputprovided from an optical head which irradiates a laser beam to, anddetects a reflected light of the irradiated laser beam from, a datarecording surface of the optical disc;

means for detecting a header mark area of the address area based on thedetection output provided from the optical head;

means for sampling and holding, based on the detection output providedfrom the header mark area detecting means, a focus error signalgenerated by the focus error generating means from the detection outputprovided from the optical head staying in the header mark area of theaddress area;

a first means for detecting a difference between the focus error signalgenerated by the focus error generating means and the focus error signalsampled and held by the sampling and holding means;

a second means for detecting a difference between the focus error signalgenerated by the focus error signal generating means and the differencedetection output provided from the first difference detecting means; and

means for controlling the focus of the optical head using as a focuserror signal the difference detection output provided from the seconddifference detecting means.

The above object can be achieved also by providing a focus controllingapparatus, for use with an optical disc having spiral grooves formed ona data recording surface thereof and in which information is written to,and read from, both a land between the grooves and the grooves astracks, each track having formed thereon a plurality of address areasand user areas, each of the address areas having formed thereon embossedpits for a groove header and embossed pits for a land header, and eachof the address areas having provided in a leading part thereof a headermark area formed from a mirror finished surface and grooves, theapparatus comprising, according to the present invention:

means for generating a focus error signal from the detection outputprovided from an optical head which irradiates a laser beam to, anddetects a reflected light of the irradiated laser beam from, a datarecording surface of the optical disc;

means for detecting a header mark area of the address area based on thedetection output provided from the optical head;

means for sampling and holding, based on the detection output providedfrom the header mark area detecting means, a focus error signalgenerated by the focus error generating means from the detection outputprovided from the optical head staying in the header mark area of theaddress area;

a first means for detecting a difference between the focus error signalgenerated by the focus error generating means and the focus error signalsampled and held by the sampling and holding means;

a second means for detecting a difference between the focus error signalgenerated by the focus error signal generating means and the differencedetection output provided from the first difference detecting means; and

means for controlling the focus of the optical head using as a focuserror signal the difference detection output provided from the seconddifference detecting means.

According to the present invention, the focus controlling apparatus maybe adapted, for instance, such that the focus controlling means controlsthe focus of the optical head using as a focus error signal the sampledand held output provided from the sampling and holding means which holdsthe difference detection output provided from the second differencedetecting means for a period during which the optical head stays in theaddress area, based on the detection output provided from the headermark area detecting means.

According to the present invention, the focus controlling apparatus mayfurther comprise, for example, a lowpass filter means which is suppliedwith the difference detection output provided from the second differencedetecting means;

the focus controlling means controlling the focus of the optical headusing as a focus error signal an output provided from the lowpass filtermeans.

The above object can be achieved also by providing a focus controllingmethod, for use with optical disc having spiral grooves formed on a datarecording surface thereof and in which information is written to, andread from, both a land between the grooves and the grooves as tracks,each track having formed thereon a plurality of address areas and userareas, each of the address areas having formed thereon embossed pits fora groove header and embossed pits for a land header, and each of theaddress areas having provided in a leading part thereof a header markarea formed from a mirror finished surface, the method comprising,according to the present invention, the steps of:

generating a focus error signal from a reflected light detection outputprovided from an optical head which irradiates a laser beam to, anddetects a reflected light of the irradiated laser beam from, a datarecording surface of the optical disc;

detecting a header mark area of the address area based on the reflectedlight detection output;

sampling and holding, based on the header mark area detection output, afocus error signal generated from the reflected light detection outputfrom the optical head staying in the header mark area of the addressarea;

firstly detecting a difference between the focus error signal generatedfrom the reflected light detection output and the sampled and held focuserror signal;

secondly detecting a difference between the focus error signal generatedfrom the reflected light detection output and the difference detectionoutput resulted from the first difference detection; and

controlling the focus of the optical head using as a focus error signalthe difference detection output resulted from the second differencedetection.

The above object can be achieved also by providing a focus controllingmethod, for use with an optical disc having spiral grooves formed on adata recording surface thereof and in which information is written to,and read from, both a land between the grooves and the grooves astracks, each track having formed thereon a plurality of address areasand user areas, each of the address areas having formed thereon embossedpits for a groove header and embossed pits for a land header, and eachof the address areas having provided in a leading part thereof a headermark area formed from a mirror finished surface and grooves, the methodcomprising, according to the present invention:

generating a focus error signal from a reflected light detection outputprovided from an optical head which irradiates a laser beam to, anddetects a reflected light of the irradiated laser beam from, a datarecording surface of the optical disc;

detecting a header mark area of the address area based on the reflectedlight detection output;

sampling and holding, based on the header mark area detection output, afocus error signal generated from the detection output provided from theoptical head staying in the header mark area of the address area;

firstly detecting a difference between the focus error signal generatedfrom the reflected light detection output and the sampled and held focuserror signal;

secondly detecting a difference between the focus error signal generatedfrom the reflected light detection output and the difference detectionoutput resulted from the first difference detection; and

controlling the focus of the optical head using as a focus error signalthe difference detection output resulted from the second differencedetection.

According to the present invention, the focus controlling method may beadapted, for example, such that based on the header mark area detectionoutput, the focus of the optical head is controlled using a focus errorsignal of which the difference detection output resulted from the seconddifference detection has been held for a period during which the opticalhead stays in the address area.

These objects and other objects, features and advantages of the presentintention will become more apparent from the following detaileddescription of the preferred embodiments of the present invention whentaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the reflected light of laser beam irradiated to thephotodetector in a conventional optical head;

FIG. 2 shows offset from an ideal pushpull signal;

FIG. 3 shows a configuration of an optical disc according to the presentinvention;

FIG. 4 shows a configuration of the optical disc in FIG. 1, showing amaddress area and user areas;

FIG. 5 shows another configuration of the optical disc in FIG. 1,showing am address area and user areas;

FIG. 6 is a schematic block diagram of a mastering apparatus formanufacture of the optical disc in FIG. 1;

FIG. 7 is a schematic block diagram of a drive for the optical disc inFIG. 1;

FIG. 8 shows a configuration of a sector recorded in the optical disc inFIG. 1;

FIG. 9 shows a configuration of an ECC block recorded in the opticaldisc in FIG. 1;

FIG. 10 shows a configuration of a frame recorded in the optical disc inFIG. 1;

FIG. 11 is a schematic block diagram of a tracking offset eliminationcircuit in a servo circuit provided in the optical disc drive in FIG. 7;

FIG. 12 is a schematic block diagram of a header mark area detector inan address circuit provided in the optical disc drive in FIG. 7;

FIG. 13 explains the operation of the header mark area detector in FIG.12;

FIG. 14 explains the operation of the tracking offset eliminationcircuit in FIG. 11;

FIG. 15 is a schematic block diagram of a focus offset eliminationcircuit in the servo circuit provided in the optical disc drive in FIG.7;

FIG. 16 explains the operation of the focus offset elimination circuitin FIG. 15;

FIG. 17 shows a still another configuration of another configuration ofthe optical disc in FIG. 3, showing am address area and user areas; and

FIG. 18 a yet another configuration of another configuration of theoptical disc in FIG. 3, showing am address area and user areas.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 3, there is illustrated an optical disc of thepresent invention. The optical disc is generally indicated with areference 1.

The optical disc 1 has formed thereon sectors by radially dividing adata recording surface of the optical disc 1 and a plurality of zones Z0to Zn by concentrically dividing the data recording surface.

Each of the sectors is assigned to an address area AR1 in which anaddress is written in a leading part thereof, and the subsequent sectorsare assigned to user areas AR2 in which data are written. In the userarea AR2, grooves GR are formed wobbling. Also in the innermost zone Z0,grooves GR are formed to wobble for a predetermined number of periods,and in the zones going towards the outer circumference of the opticaldisc, grooves GR are formed to wobble for an increased number ofwobbling periods. The address areas AR1 are discretely disposed on theoptical disc and have address information recorded for each sectortherein. The address area AR1 has assigned thereto a lengthcorresponding to the predetermined number of wobbling periods of thegroove GR.

As shown in FIG. 4, in a former half of the address area AR1, a row ofembossed address pits for a groove header GRH is formed on a trackcenter line through the groove GR. In latter half of the address areaAR1, a row of embossed address pits for a land header LH is formed on atrack center line through a land.

Note that the embossed address pits formed in the address area AR1 maybe disposed in positions adjoining the groove track and land track,respectively, as shown in FIG. 5.

In the leading area of the address area AR1, a header mark HM is formedfrom a mirror finished surface, not by grooving and embossing. Theheader mark HM formed from the mirror finished surface is higher inreflectivity than the user areas AR2 having land and groove tracks andeach address pit area including the land header LH and groove header GRHin the address area AR1, and thus it can easily be detected.

The optical disc 1 is manufactured using a disc master prepared by amastering apparatus 10 configured as shown in FIG. 6, for example.

The mastering apparatus 10 comprises a wobbling signal generationcircuit 11, an address signal generation circuit 12, and a synthesiscircuit 13 supplied with signals from the wobbling signal generationcircuit 11 and address signal generation circuit 12, respectively, toprovide a drive signal SD by synthesis, a drive circuit 14 to drive anoptical head 15 based on the drive signal supplied from the synthesiscircuit 13, a spindle motor 17 to drive to spin a disc master 16, and asystem control circuit (not shown).

The wobbling signal generation circuit 11 provides as a wobbling signalWB a sinewave signal of a predetermined frequency synchronous with thespinning of the disc master 16. Also, this circuit 11 stepwise increasesthe frequency of the wobbling signal WB correspondingly to a zoning andprovides it as an output. The circuit 11 shifts the laser beam focusposition to wobble the groove GR for a predetermined period per sectoraccording to the wobbling signal WB.

The address signal generation circuit 12 is controlled by the systemcontrol circuit (not illustrated) to generate an address signal SA ofwhich the value is sequentially changed according to the displacement ofthe optical head 15.

More specifically, the address signal generation circuit 12 is suppliedwith timing signals including an FG signal synchronous with the spinningof the disc master 16, etc. from the spindle motor 17, etc. and countsthe timing signals by a predetermined counter to generate an addressdata ID indicative of the laser beam focus position. At this time, theaddress signal generation circuit 12 adds to the address data ID asector mark SM, sync timing data VFO, header mark AM and a postamble PAto generate a groove header and land header for assignment to the formerand latter halves of the address area AR1. Note that a same clockpattern as a one written to, and read from, a leading part of the userarea AR2 is generated as the sync timing data VFO. The address signalgeneration circuit 12 converts a sector header thus generated to astring of serial data, and modulates the serial data string into apredetermined format.

Further, the address signal generation circuit 12 supplies the synthesiscircuit 13 with the modulation output as an address signal SA. Theaddress signal generation circuit 12 provides the address signal SA at atime corresponding to the scanning with a laser beam L from the opticalhead 15.

The drive circuit 14 zones the disc master 16 by changing the drivingconditions for the optical head 15 at a time synchronous with thespinning of the disc master 16 correspondingly to the laser beam focusposition. More particularly, the drive circuit 14 changes the drivingconditions for the optical head 15 in such a manner as to radiallydivide the data recording surface of the disc master 16 to form sectors.Further, the time of changing the driving conditions is shifted stepwisefrom the inner circumference of the disc master 16 towards the outercircumference to stepwise divide the data recording surfaceconcentrically, to thereby form a plurality of zones Z0 to Zn.

Also, the drive circuit 14 is controlled by the system control circuit(not shown) to shift the laser beam focus position in the user area AR2under the drive signal SD, thereby forming a wobbling groove GR in theuser area AR2.

In the former half of the address area AR1, the laser beam focusposition shifting is ceased and the amount of laser beam isintermittently increased under the drive signal SD, to thereby form arow of pits on a track center line through the groove GR. In the latterhalf of the address area AR1, the laser beam focus position is shiftedonto a track center line through the inner land and the amount of laserbeam is intermittently increased under the drive signal SD, to therebyform a row of pits on the track center line through the land.

At this time, the drive circuit 14 allows to record, in the former halfof the address area AR1, the address data recorded in the sector by thegroove GR following the sector in the form of the pit row on the trackcenter line, and in the latter half of the address area AR1, the addressdata recorded in the sector by an inner land following the sector in theform of the pit row on the track center line.

The optical head 15 is constructed to be moved radially of the discmaster 16 by the optical system. Further, the optical head 15 has thefocused spot diameter of the laser beam L set in manufacture of anoptical disc using the disc master 16 so that the grooves GR formed byexposure to the laser beam L have a same width as the land formedbetween the grooves GR. In this embodiment, the spot shape and lightamount of the laser beam L are set for the effective exposure range tothe laser beam L to be larger than a final intended width of the grooveGR. Therefore, the optical head 15 allows to expose the disc master 16so that an optical disc produced using the disc master 16 can have datarecorded on the land and in the grooves GR.

The disc master 16 is formed from a glass substrate having a photoresistcoating applied to a surface thereof, for example. The disc master 16 isdriven by the spindle motor 17 to spin at a constant angular velocity.The optical head 15 irradiates a laser beam L onto the disc master 16while being moved by a predetermined sled mechanism little by littleradially from the inner circumference towards the outer circumference ofthe disc master 16 synchronously with the spinning of the disc master16. Thus the optical head 15 allows to form spiral tracks one afteranother from the inner circumference towards outer circumference of thedisc master 16.

Through the aforementioned procedure, the mastering apparatus 10produces the above-mentioned optical disc 1 which is a high NA onehaving a thickness of 0.1 mm and a numerical aperture (NA) as high as0.85, for example, by irradiation of a laser beam L to the disc master16.

Next, an optical disc recording/reproducing apparatus (will be referredto as “optical disc drive” hereinunder) 20 destined to write and readdata to and from the optical disc 1, will be described herebelow withreference to FIG. 7.

The optical disc drive 20 comprises an optical head 21 to write and readdata by means of a laser beam, a write/read circuit 22 to convert thedata to a predetermined format, a sled motor 23 to move the optical head21 radially of the optical disc 1, a servo circuit 24 to controltracking and focus of he optical head 21, an address detection circuit25 to detect an address of the data being read out from the optical disc1, and a spindle motor 26 to drive to spin the optical disc 1.

The optical head 21 writes a data supplied via a system control circuit32 which will further be described later and the write/read circuit 22to the optical disc 1 by means of a laser beam. The write/read circuit22 converts a data supplied from the system control circuit 32 to apredetermined format, and provides it to the optical head 21.

The write/read circuit 22 converts the data for each sector configuredas shown in FIG. 8, and writes it to the optical disc 1 by means of theoptical head 21. As shown, the sector consists of a header area formedfrom embossed pits and a write/read area of 8 kB.

The header consists of a sector mark (SM), sync timing data VFO1 andVFO2, header marks (AM1 and AM2), ID1 and ID2, and postambles (PA1 andPA2). The groove has a header 1 formed therein, and the land has aheader 2 formed thereon. Gap areas (Gap1, Gap2 and Gap3) are used tochange the timing signal. Guard areas (Guard1 and Guard2) are used forstarting and ending a writing. The guard area has a predetermined lengthand shifts the position of a written data by j-ch each time data iswritten. Thus, data at write starting and ending positions are not usedany longer, so that deterioration of record identification at the writestarting and ending positions due to an overwrite will not adverselyaffect on the data. The position of written data is shifted at each datawriting, which improves the number of times of overwriting. At the VFOs,PLL (phase locked loop) is pulled in. SYNC indicates a starting positionof a data. Buffer area is to absorb an influence of a jitter due to suchas run-out or eccentricity. In one sector, a groove is formed to have408 wobbling periods.

On the other hand, each error correction block (ECC block) is composedof 64 kB as shown in FIG. 9. It may be also handled as a 2-kB datasector for write and read. In this case, each error correction block iswritten and read with 64 kB, and a desired data of 2 kB is to be writtenand read to and from the error correction block. More particularly, aparity of 24 bytes is added to a data of 172 bytes. Note that theinterleave length is 384 bytes.

FIG. 10 shows the configuration of a frame. As shown, a frame sync islocated along with a dcc bit at the top of each frame. Frame data is aquarter of a data in one interleave of the error correction block. Eachframe consists of 16 dcc blocks. The dcc block consists of a {fraction(1/16)} of a frame data and a dcc.

The optical head 21 is disposed opposite a high NA optical disc havingan NA of 0.85 and thickness of 0.1 mm, and uses a laser of 635 nm or 515nm in wavelength. The optical head 21 supplies a detection output of areflected light of the laser beam from the optical disc 1 to thewrite/read circuit 22, servo circuit 24, address detection circuit 25and wobbling signal generation circuit 27.

The servo circuit 24 generates a focus error signal from the detectionoutput provided from the optical head 21, and controls the focus of theoptical head 21 based on the focus error signal. The servo circuit 24generates a pushpull signal from the detection output provided from theoptical head 21, and controls the tracking of the optical head 21 basedon the pushpull signal.

The servo circuit 24 has a tracking error offset elimination circuit 40constructed as shown in FIG. 11, and supplies the pushpull signal tothis tracking error offset elimination circuit 40 in which an offset iseliminated from the pushpull signal to provide a tracking error signalunder which the tracking of the optical head 21 is controlled.

More particularly, the tracking error offset elimination circuit 40comprises a sample and hold circuit 42 to sample and hold a pushpullsignal supplied as a tracking error signal at an input terminal 41thereof, a differential amplifier 43 to detect a difference between apushpull signal from the input terminal 41 and a one from the sample andhold circuit 42, and a sample and hold circuit 44 to sample and hold apushpull signal from the differential amplifier 43. The sample and holdcircuit 42 samples and holds the pushpull signal synchronously with asample and hold pulse (will be referred to as “S/H pulse” hereinunder) Bwith the sampling and holding time constants being kept unchanged untilthe laser beam moves over one sector. The sample and hold circuit 44samples and holds the pushpull signal synchronously with an S/H pulse Awith the sampling and holding time constants being kept unchanged untilthe laser beam moves over the address area AR1.

The S/H pulses A and B are produced by a timing generator (not shown)based on a detection output provided from a header mark detector 50. Theheader mark detector 50 comprises a level comparator 51, as shown inFIG. 12, to detect a header mark area HM formed from a mirror finishedsurface of a leading portion of each address area. The header mark HMformed from the mirror finished surface as mentioned above has a higherreflectivity than each address pit area of the land header LH and grooveheader GRH of the user areas AR2 and address area AR1 of the land trackand groove track. Thus the header mark area detector 50 compares, in thelevel comparator 51, a signal level of a sum signal SUM obtained as adetection output provided from the optical head 21 or its lowpass filteroutput LPFsum signal, with a reference level Ref as shown in FIG. 13 todetect the header marker HM simply and positively. The header mark areadetector 50 is provided in the address detection circuit 25, forexample.

The sample and hold circuit 42 samples and holds a pushpull signalsynchronously with the S/H pulse B to detect a direct-current (DC)offset of the pushpull signal and holds it for a period of one sector.When the laser beam is incident upon the user area AR2 through theaddress area AR1, the differential amplifier 43 eliminates or removesthe DC offset from the pushpull signal from the user area AR2 andprovides an offset-free pushpull signal.

As shown in FIG. 14, the tracking error offset elimination circuit 40detects a header mark HM in the address area AR1 of each sector, samplesand holds an offset of a pushpull signal, that is, a tracking errorsignal based on the detection output, and eliminates the offset from thepushpull signal in the user area AR2, thereby providing an appropriatepushpull signal. Thus, even if a run-out or eccentricity takes place inthe spinning of the optical disc 1 and a DC offset is arisen in thepushpull signal, the optical head tracking can be controlled accurately.

Since the tracking error offset elimination circuit 40 cannot provide anormal pushpull signal, the sample and hold circuit 44 samples and holdsa pushpull signal in a user area AR2 just before the address area AR1,thereby providing a pushpull signal.

Note that the sample and hold circuit 44 may be replaced with a lowpassfilter since it is destined for removing a disturbance in the addressarea AR1.

At the stage where the tracking servo is pulled in, no servo loop hasyet been formed. So, based on the detection output provided from theheader mark area detector 50, the tracking servo pull-in with theoptical head standing in the address area is disabled and only atracking servo pull-in with the optical head standing in the user areais enabled. Thus, the tracking servo pull-in can be quickly andcorrectly.

Even when the laser beam is focused on a wobbling land, the trackingerror offset elimination circuit 40 can eliminate an offset havingtaking place in the pushpull signal as in the above.

The servo circuit 24 has a focus error offset elimination circuit 60constructed, as shown in FIG. 15, to eliminate an offset from a pushpullsignal, thereby producing a focus error signal which is used to controlthe focus of the optical head 21.

More particularly, the focus error offset elimination circuit 60comprises a sample and hold circuit 62 to sample and hold a focus errorsignal supplied at an input terminal 61 thereof, a first differentialamplifier 63 to detect, as a focus error offset, a difference between afocus error signal from the input terminal 61 and a one from the sampleand hold circuit 62, a second differential amplifier 64 to detect adifference between a focus error signal supplied at the input terminal61 and the focus error offset detected by the first differentialamplifier 63, and a sample and hold circuit 65 to sample and hold adifference detection output provided from the second differentialamplifier 64.

The sample and hold circuit 62 samples and holds the focus error signalsynchronously with the S/H pulse B. And the sample and hold circuit 65samples and holds the difference detection output synchronously with theS/H pulse A.

The sample and hold circuit 62 samples and holds the focus error signalsynchronously with the S/H pulse B to hold it for a period of onesector. The focus error signal, that is, a focus error signal in theheader mark HM formed from the mirror finished surface provided in theleading part of the address area AR1, indicates a focus error notinfluenced by the land and groove, that is, a focus error free from thefocus error offset. As shown in FIG. 16, the first differentialamplifier 63 detects, as a focus error offset Δ, a difference betweenthe focus error signal sampled and held by the sample and hold circuit62 and a focus error caused with the optical head staying in the userarea AR2. The second differential amplifier 64 detects a differencebetween a focus error signal supplied at the input terminal 61 and afocus error offset detected by the first differential amplifier 63,thereby generating a focus error signal free from the focus erroroffset.

In the focus error offset elimination circuit 60, a focus error signalfrom which the focus error offset has been removed is further sampledand held by the sample and hold circuit 65 synchronously with the S/Hpulse A, to thereby eliminate a disturbance in the address area AR1.

Note that since the sample and hold circuit 65 is provided to remove thedisturbance in the address area AR1, it may be replaced with a lowpassfilter.

On the other hand, the address detection circuit 25 in the optical discdrive 20 shown in FIG. 7 decodes an address data from the detectionoutput provided from the optical head 21 for detection of any error orother, and then supplies the decoded data to the system control circuit32.

The optical disc drive 20 comprises a wobbling signal generation circuit27, wobbling period detection circuit 28, PLL (phased-locked loop)circuit 29, cluster counter 30 to count the address position, ROM 31having a predetermined control program stored therein, and a systemcontrol circuit 32 to control each circuit in the optical disc drive 20.

The wobbling signal detection circuit 27 comprises a bandpass filter(BPF) 27 a to remove a noise component from the wobbling signal, and acomparator 27 b for binary coding of the wobbling signal. The BPF 27 ais supplied via the optical head 21 with a groove wobbling detectionsignal (wobbling signal) recorded in the optical disc 1, and removes anoise from the wobbling signal. The noise-free wobbling is supplied tothe comparator 27 b which will binary-code the wobbling signal from theBPF 27 a to produce a wobbling detection pulse which will be supplied tothe wobbling period detection circuit 28.

The wobbling period detection circuit 28 judges whether the wobblingdetection pulse has a predetermined periodicity. If the pulse is judgedto have the periodicity, it is supplied to the PLL circuit 29.

The PLL circuit 29 comprises a phase comparator 29 a, lowpass filter(LPF) 29 b to remove a high frequency noise component, voltage controloscillator (VCO) 29 c and a frequency divider 29 d.

The phase comparator 29 a compares the phase of a wobbling detectionpulse from the wobbling period detection circuit 28 with that of a pulsefrom the frequency divider 29 d to supply a phase comparison errorsignal indicative of a phase error to the VCO 29 c via the LPF 29 b. TheVCO 29 c generates a channel clock (will be referred to as “R/W clock”hereinunder) based on the phase comparison error signal, and supplies itto the frequency divider 29 d and cluster counter 30. The frequencydivider 29 d has the frequency division ratio controlled by the systemcontrol circuit 32, and divides the frequency of the R/W clock from theVCO 29 c to generate a pulse having a same frequency as the wobblingsignal for supply to the phase comparator 29 a. With these operations,an R/W clock is generated based on the wobbling signal.

Based on an address from the address detection circuit 25 and an R/Wclock from the VCO 29 c, the cluster counter 30 counts the position of anext address by generating a sync signal synchronous with the period ofthe address from the address detection circuit 25. Also, when thecluster counter 30 cannot detect the position of the next address, theposition of the next address is determined and the address is countedup.

The system control circuit 32 controls the revolution of the sled motor23 based on the address detected by the cluster counter 30, and allowsthe optical head 21 to access to a predetermined position on the opticaldisc 1 and write or read data to or from the optical disc 1 at thecluster write/read timing. Also, the ROM 31 has stored therein afrequency division ratio data for the frequency divider 29 d for eachaddress. The system control circuit 32 controls the frequency divisionratio for the frequency divider 29 d based on the data in the ROM 31.The system control circuit 32 controls the write/read circuit 22,processes a data read by the optical head 21 in a predetermined manner,provides the processed data to outside, and converts an external inputdata to a predetermined format. The converted data is written to theoptical disc 1 by means of the optical head 21.

In the embodiment having been described in the foregoing, the headermark area HM is formed of a mirror finished surface in the leading partof the address area AR1 as shown in FIGS. 4 and 5. However, the presentinvention is not limited to this optical disc configuration.

As shown in FIGS. 17 and 18, a header mark HM consisting of a mirrorfinished surface M and groove G may be formed in the leading header markarea of each address area. In this case, the header mark HM in theheader mark area may be formed to indicate a boundary between a groovetrack and land track with a unique pattern. In the optical disc drive20, the header mark HM formed to indicate the boundary between thegroove and land tracks with the unique pattern is detected to change thepolarity of a tracking error signal.

As having been described in the foregoing, the optical disc according tothe present invention has spiral grooves formed on a data recordingsurface thereof and in which information is written to, and read from,both a land between the grooves and the grooves as tracks, each trackhaving formed thereon a plurality of address areas and user areas, eachof the address areas having formed thereon embossed pits for a grooveheader and embossed pits for a land header, and each of the addressareas having provided in a leading part thereof a header mark areaformed from a mirror finished surface, whereby offset arisen in trackingand focus error signals can be eliminated to accurately control thetracking and focus.

Also, the optical disc according to the present invention has spiralgrooves formed on a data recording surface thereof and in whichinformation is written to, and read from, both a land between thegrooves and the grooves as tracks, each track having formed thereon aplurality of address areas and user areas, each of the address areashaving formed thereon embossed pits for a groove header and embossedpits for a land header, and each of the address areas having provided ina leading part thereof a header mark area formed from a mirror finishedsurface and grooves, whereby offset arisen in tracking and focus errorsignals can be eliminated to accurately control the tracking and focus.

The optical disc according to the present invention has, for instance,groove tracks and land tracks formed to wobble at a single frequency,whereby the single frequency may be given as a reference frequency tothe reproducing system.

The optical disc according to the present invention is formed, forinstance, to have a plurality of zones formed by dividing the datarecording surface of the optical disc concentrically and a plurality ofsector structures formed by dividing each zone on the data recordingsurface into radial areas.

The optical disc according to the present invention has the header markin the header mark area formed to indicate a boundary between a groovetrack and land track with a unique pattern.

According to the present invention, the tracking controlling apparatusand method, for use with an optical disc having spiral grooves and landsformed on a data recording surface thereof and in which information iswritten to, and read from, both a land between the grooves and thegrooves as tracks, each track having a plurality of address areas anduser areas formed thereon, each of the address areas having an embossedpit for a groove header and an embossed pit for a land header, and eachof the address areas having formed in a leading part thereof a headermark area consisting of a mirror finished surface, are adapted such thata tracking error signal is generated by a tracking error generatingmeans from a detection output provided from an optical head whichirradiates a laser beam to, and detects a reflected light of theirradiated laser beam from, a data recording surface of the opticaldisc; a header mark area of the address area is detected by a headermark area detecting means based on the detection output provided fromthe optical head; a tracking error signal generated by the trackingerror generating means from the detection output provided from theoptical head staying in the header mark area of the address area issampled and held by the sampling and holding means to detect a trackingerror offset as a sampled and held output provided from the sampling andholding means; and a difference between the tracking error signalgenerated by the tracking error generating means and the tracking errorsignal sampled and held by the sampling and holding means is detected bya difference detecting means to provide a tracking error signal fromwhich the tracking error offset has been removed. Therefore, thedifference detection output provided from the difference detecting meanscan be used to accurately control the tracking of the optical head.

The tracking controlling apparatus and method according to the presentinvention is adapted, for example, such that the tracking controllingmeans disables a tracking servo pull-in with the optical head standingin the address area and enables only a tracking servo pull-in with theoptical head standing in the user area, based on the detection outputprovided from the header mark area detecting means. Thus the trackingservo can positively be pulled in.

The tracking controlling apparatus and method according to the presentinvention is adapted, for instance, such that based on the detectionoutput provided from the header mark area detecting means, the trackingcontrolling means controls the tracking of the optical head using as atracking error signal a sampled and held output provided from thesampling and holding means which holds the difference detection outputprovided from the difference detecting means for a period during whichthe optical head stays in the address area, to eliminate a disturbancein the address area from the tracking error signal. Thus the tracking ofthe optical head can accurately be controlled by the trackingcontrolling means.

The tracking controlling apparatus and method according to the presentinvention is adapted, for example, such that the tracking controllingmeans controls the tracking of the optical head using as a trackingerror signal an output provided from a lowpass filter means which issupplied with the difference detection output provided from thedifference detecting means, to remove a disturbance in the address areafrom the tracking error signal. Thus the tracking of the optical headcan accurately be controlled by the tracking controlling means.

In the focus controlling apparatus and method according to the presentinvention, for use with an optical disc having spiral grooves and landsformed on a data recording surface thereof and in which information iswritten to, and read from, both a land between the grooves and thegrooves as tracks, each track having a plurality of address areas anduser areas formed thereon, each of the address areas having an embossedpit for a groove header and an embossed pit for a land header, and eachof the address areas having formed in a leading part thereof a headermark area consisting of a mirror finished surface and grooves; a focuserror signal is generated from a detection output provided from anoptical head which irradiates a laser beam to, and detects a reflectedlight of the irradiated laser beam from, a data recording surface of theoptical disc; a header mark area of the address area is detected by theheader mark area detecting means based on the detection output providedfrom the optical head; a focus error signal generated by the focus errorgenerating means from the detection output provided from the opticalhead staying in the header mark area of the address area is sampled andheld by a sampling and holding means based on the detection outputprovided from the header mark area detecting means; a difference betweenthe focus error signal generated by the focus error generating means andthe focus error signal sampled and held by the sampling and holdingmeans is detected by a first difference detecting means; a differencebetween the focus error signal generated by the focus error signalgenerating means and the difference detection output provided from thefirst difference detecting means is detected by a second differencedetecting means. Thus a focus error signal can be obtained from which afocus error offset has been eliminated. The difference detection outputprovided from the second difference detecting means can be used as afocus error signal to accurately control the focus of the optical head.

The focus controlling apparatus and method according to the presentinvention is adapted, for instance, such that based on the detectionoutput provided from the header mark area detecting means, a sampled andheld output provided from the sampling and holding means which holds thedifference detection output provided from the second differencedetecting means for a period during which the optical head stays in theaddress area, is used as a focus error signal to eliminate a disturbancein the address area from the focus error signal. Thus the focus of theoptical head can accurately be controlled by the focus controllingmeans.

The focus controlling apparatus and method according to the presentinvention is adapted, for example, such that an output provided from alowpass filter means which is supplied with the difference detectionoutput provided from the second difference detecting means, is used as afocus error signal to remove a disturbance in the address area from thefocus error signal. Thus the focus of the optical head can accurately becontrolled by the focus controlling means.

What is claimed is:
 1. A focus controlling apparatus, for use with anoptical disc having spiral grooves formed on a data recording surfacethereof and in which information is written to, and read from, both aland between the grooves and the grooves as tracks, each track havingformed thereon a plurality of address areas and user areas, each of theaddress areas having formed thereon embossed pits for a groove headerand embossed pits for a land header, and each of the address areashaving provided in a leading part thereof a header mark area formed froma mirror finished surface, the apparatus comprising: means forgenerating a focus error signal from a detection output provided from anoptical head which irradiates a laser beam to, and detects a reflectedlight of the irradiated laser beam from, a data recording surface of theoptical disc; means for detecting a header mark area of the address areabased on the detection output provided from the optical head; means forsampling and holding, based on the detection output provided from theheader mark area detecting means, a focus error signal generated by thefocus error generating means from the detection output provided from theoptical head staying in the header mark area of the address area; afirst means for detecting a difference between the focus error signalgenerated by the focus error generating means and the focus error signalsampled and held by the sampling and holding means; a second means fordetecting a difference between the focus error signal generated by thefocus error signal generating means and the difference detection outputprovided from the first difference detecting means; and means forcontrolling the focus of the optical head using as a focus error signalthe difference detection output provided from the second differencedetecting means.
 2. The apparatus as set forth in claim 1, furthercomprising means for sampling and holding, based on the detection outputprovided from the header mark area detecting means, the differencedetection output provided from the header mark area detecting means fora period during which the optical head stays in the address area; thefocus controlling means controlling the focus of the optical head usingthe sampled and held output provided from the sampling and holding meansas a focus error signal.
 3. The apparatus as set forth in claim 1,further comprising a lowpass filter means which is supplied with thedifference detection output provided from the second differencedetecting means; the focus controlling means controlling the focus ofthe optical head using an output provided from the lowpass filter meansas a focus error signal.
 4. A focus controlling method, for use with anoptical disc having spiral grooves formed on a data recording surfacethereof and in which information is written to, and read from, both aland between the grooves and the grooves as tracks, each track havingformed thereon a plurality of address areas and user areas, each of theaddress areas having formed thereon embossed pits for a groove headerand embossed pits for a land header, and each of the address areashaving provided in a leading part thereof a header mark area formed froma mirror finished surface, the method comprising the steps of:generating a focus error signal from a detection output provided from anoptical head which irradiates a laser beam to, and detects a reflectedlight of the irradiated laser beam from, a data recording surface of theoptical disc; detecting a header mark area of the address area based onthe reflected light detection output; sampling and holding, based on theheader mark area detection output, a focus error signal generated fromthe reflected light detection output provided from the optical headstaying in the header mark area of the address area; firstly detecting adifference between the focus error signal generated from the reflectedlight detection output and the sampled and held focus error signal;secondly detecting a difference between the difference detection outputgenerated from the reflected light detection output and the differencedetection output obtained by the first difference detection; andcontrolling the focus of the optical head using as a focus error signalthe difference detection output obtained from the second differencedetection.
 5. The method as set forth in claim 4, wherein the focus ofthe optical head is controlled using the focus error signal held for aperiod during which the optical head stays in the address area based onthe header mark area detection output.
 6. A focus controlling apparatus,for use with an optical disc having spiral grooves formed on a datarecording surface thereof and in which information is written to, andread from, both a land between the grooves and the grooves as tracks,each track having formed thereon a plurality of address areas and userareas, each of the address areas having formed thereon embossed pits fora groove header and embossed pits for a land header, and each of theaddress areas having provided in a leading part thereof a header markarea formed from a mirror finished surface and grooves, the apparatuscomprising: means for generating a focus error signal from a detectionoutput provided from an optical head which irradiates a laser beam to,and detects a reflected light of the irradiated laser beam from, a datarecording surface of the optical disc; means for detecting a header markarea of the address area based on the detection output provided from theoptical head; means for sampling and holding, based on the detectionoutput provided from the header mark area detecting means, a focus errorsignal generated by the focus error generating means from the detectionoutput provided from the optical head staying in the header mark area ofthe address area; a first means for detecting a difference between thefocus error signal generated by the focus error generating means and thefocus error signal sampled and held by the sampling and holding means; asecond means for detecting a difference between the focus error signalgenerated by the focus error signal generating means and the differencedetection output provided from the first difference detecting means; andmeans for controlling the focus of the optical head using as a focuserror signal the difference detection output provided from the seconddifference detecting means.
 7. The apparatus as set forth in claim 6,further comprising: means for sampling and holding, based on thedetection output provided from the header mark area detecting means, thedifference detection output provided from the second differencedetecting means for a period during which the optical head stays in theaddress area; the focus controlling means controlling the focus of theoptical head using as a focus error signal the sampled and held outputprovided from the sampling and holding means.
 8. The apparatus as setforth in claim 6, further comprising: a lowpass filter means which issupplied with the difference detection output provided from the seconddifference detecting means; the focus controlling means controlling thefocus of the optical head using the output provided from the lowpassfilter means as a focus error signal.
 9. A focus controlling method, foruse with an optical disc having spiral grooves formed on a datarecording surface thereof and in which information is written to, andread from, both a land between the grooves and the grooves as tracks,each track having formed thereon a plurality of address areas and userareas, each of the address areas having formed thereon embossed pits fora groove header and embossed pits for a land header, and each of theaddress areas having provided in a leading part thereof a header markarea formed from a mirror finished surface and grooves, the methodcomprising the steps of: generating a focus error signal from adetection output provided from an optical head which irradiates a laserbeam to, and detects a reflected light of the irradiated laser beamfrom, a data recording surface of the optical disc; detecting a headermark area of the address area based on the reflected light detectionoutput; sampling and holding, based on the header mark area detection, afocus error signal generated from the reflected light detection outputwith the optical head in the header mark area of the address area;firstly detecting a difference between the focus error signal generatedfrom the reflected light detection output and the sampled and heldtracking error signal; secondly detecting a difference between the focuserror signal generated from the reflected light detection output and thedifference detection output obtained by the first difference detection;and controlling the focus of the optical head using as a tracking errorsignal the difference detection output obtained by the second differencedetection.
 10. The method as set forth in claim 9, wherein the focus ofthe optical head is controlled using the focus error signal of which thedifference detection output obtained by the second difference detectionhas been held for a period during which the optical head stays in theaddress area.